1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED) for realizing a full-color display device and, more particularly, to a polymer OLED which can increase uniformity in thickness of a layer formed of polymer ink and can prevent mixing of colors.
2. Description of the Related Art
OLEDs which can realize full-color display devices are largely divided into two types according to the organic material used: OLEDs employing low molecular materials; and OLEDs employing polymer high molecular materials.
A high molecular OLED is generally fabricated such that two opposite electrodes, that is, a cathode and an anode, are disposed on a substrate, and a hole transport layer (HTL) and an emission layer are provided between the anode and the cathode. In the polymer high molecular OLED, the HTL and the emission layer are formed of organic polymers. Recently, research into polymer OLEDs has been actively carried out because they are driven at a relatively lower voltage, consume small power, and can easily realize large, full-color display screens.
Organic layers, both as active and passive matrix types on the basis of polymer OLEDs, are fabricated according to the state of the art with printing techniques such as ink-jet printing.
In this known method, the light emitting polymers are liquified to form a so-called polymer ink. The polymer ink is printed onto a substrate via an ink-jet printing head.
The OLED, in the simplest case of a passive matrix display screen, is fabricated as follows.
First, a transparent substrate made from glass or plastic is coated with a transparent conductor material, such as indium tin oxide (ITO), to form an anode having a predetermined pattern.
In the next step, a hole transport layer (HTL) is formed of an organic material, for example, poly-(2,4)-ethylene-dihydroxy thiophene (PEDOT) or polyaniline (PANI). The HTL is deposited on the anode on the substrate by means of ink-jet printing or spin coating.
A polymer emission layer is then formed on the organic HTL by the ink-jet printing method as mentioned above. In order to obtain a full-color display screen, a red-emitting, a green-emitting and a blue-emitting polymer are imprinted. Following this, a cathode, e.g., from a layer of calcium and a layer of aluminum, is vapor-deposited.
Finally, the entire structural element is encapsulated. The cathode and the anode are then connected to an electronic driving system.
In producing the OLED, in order to print the polymers for each pixel, a partition structure for preventing ink containing various colors from penetrating into neighboring pixels is necessary. The partition structure is largely divided into two types according to printing methods.
First, an organic layer may be formed at an emitting region only by accurately dropping an inklet for each sub-pixel of a pixel. In this case, a partition structure is provided to define sub-pixels. In order to form an organic layer at an emitting region only, it is necessary to drop an inklet into each sub-pixel accurately, resulting in prolonged processing time. Also, since high accuracy is required, defects may be easily generated.
Alternatively, rather than forming an organic layer only at an emitting region, all pixels may be covered by an organic layer. According to this method, since it is not necessary to limit a pixel to a predetermined size, a reduction in aperture ratio can be prevented. In this case, in addition to a partition structure for defining the respective sub-pixels, another partition structure for separating pixels having various colors from one another is also provided. Such a partition structure for separating pixels having various colors from one another is generally formed in a line array.
For example, U.S. Pat. No. 6,388,377 to Kobayashi, entitled  ELECTROLUMINESCENT ELEMENT WITH BANKS INTERSECTING ANODE GROUP , issued on May 14, 2002, discloses a passive matrix type OLED in which line-patterned channels are formed to be orthogonal to an anode and an emission layer and a cathode are defined by the channels. U.S. Pat. No. 6,380,672 to Yudasaka, entitled ACTIVE MATRIX DISPLAY DEVICE, issued on Apr. 30, 2002, and U.S. Pat. No. 6,373,453 to Yudasaka, entitled ACTIVE MATRIX DISPLAY, issued on Apr. 16, 2002, disclose active matrix type OLEDs in which a partition structure is formed at a TFT region excluding pixel regions where light emission occurs. Also, the fabrication of organic light emitting diodes on a polymer basis is described in the European Patent Publication No. 0423283 B1 to Fiend, entitled ELECTROLUMINESCENT DEVICES, published on 25 Jan. 1995 and PCT publication No. WO 9013148 to Fiend et al., entitled ELECTROLUMINESCENT DEVICES, published on 1 Nov. 1990. The fabrication of OLEDs by means of printing methods, such as ink-jet printing, is described in the following U.S., European and PCT Publications: European Patent Publication No. 0908725 A1 to Fukushima et al., entitled MANUFACTURE OF A MICROSENSOR DEVICE, published on 14 Apr. 1999, European Patent Publication No. 0940796 A1 to Yudasaka, entitled ACTIVE MATRIX DISPLAY, published on 8 Sep. 1999, European Patent Publication No. 0940797 A1 to Yudasaka, entitled ACTIVE MATRIX DISPLAY, published on 8 Sep. 1999, European Patent Publication No. 0989778 A1 to Kiguchi, entitled SUBSTRATE FOR PATTERNING THIN FILM AND SURFACE TREATMENT THEREOF, published on 29 Mar. 2000; PCT publication No. WO 9943031 to Friend et al, entitled DISPLAY DEVICES, published on 26 Aug. 1999, PCT publication No. WO 9966483 to Heeks et al., entitled BACKLIT DISPLAYS, published on 23 Dec. 1999, PCT publication No. WO 9828946 to Thompson et al., entitled MULTICOLOR DISPLAY DEVICES, published on 2 Jul. 1998, U.S. Pat. No. 6,087,196 to Sturm et al., entitled FABRICATION OF ORGANIC SEMICONDUCTOR DEVICES USING INK JET PRINTING, issued on Jul. 11, 2000, PCT publication No. WO 0012226 to Jones et al., entitled FULL COLOR ORGANIC LIGHT EMITTING DIODE DISPLAY AND METHOD FOR MAKING THE SAME USING INKJET FABRICATION, published on 9 March 2000, and PCT publication No. WO 0019776 to Young et al., entitled METHOD AND DEVICE FOR MANUFACTURING AN ELECTROLUMINESCENT DISPLAY SCREEN, published on 6 Apr. 2000.
In such prior arrangements and methods, the HTL and the emission layer exhibit nonuniformity in layer thickness at the upper and lower rims thereof, which are effective areas for electro-luminescence emission. This occurs because the layer thicknesses of the HTL and polymer emission layer gradually become thinner, or the HTL ink and polymer ink run out from the channels.
In addition to the problem of nonuniformity in layer thickness, ink may run out into neighboring channels through walls of the channels, causing mixing of colors.